1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with an antiparallel (AP) pinned structure.
2. Background of the Invention
One type of conventional magnetoresistive sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV magnetoresistive sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor. CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002.
One type of CPP-SV sensor used in read heads includes an antiparallel (AP) pinned structure. The AP-pinned structure has first (AP1) and second (AP2) ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. The AP2 layer, which is in contact with the nonmagnetic APC layer on one side and the sensor's Cu spacer on the other side, is typically referred to as the reference layer. The AP1 layer, which is typically in contact with an antiferromagnetic or hard magnet pinning layer on one side and the nonmagnetic APC layer on the other side, is typically referred to as the pinned layer. If the AP-pinned structure is the “self-pinned” type, then no pinning layer is required. In a self-pinned structure where no antiferromagnet or hard magnet pinning layer is present, the AP1 layer is in contact with a seed layer on the sensor substrate. The AP-pinned structure minimizes magnetostatic coupling between the reference layer and the CPP-SV free ferromagnetic layer. The AP-pinned structure, also called a “laminated” pinned layer, and sometimes called a synthetic antiferromagnet (SAF), is described in U.S. Pat. No. 5,465,185.
The magnetoresistance (ΔR/R) of a CPP-SV read head can be increased by increasing the thickness of the reference ferromagnetic (AP2) layer to generate more electron spin-dependent scattering within the bulk of the AP2 layer. The spin-diffusion length for typical CoFe and NiFe alloys is greater than the typical thickness of AP2, which is about 15-45 Å. If AP1 and AP2 are made of the same ferromagnetic material and the thickness of AP2 is increased, the thickness of AP1 also has to be increased to cancel out the magnetic stray fields originating from the two AP layers acting on the free layer and to assure that AP1 and AP2 have similar but not equal magnetic moments. The reason for this is to keep the net magnetic moment of the AP-pinned structure small so that the magnetostatic coupling field between the pinned and free layer is small. However, increasing the magnetic moments of AP1 and AP2 will decrease the saturation field Hs of the AP-pinned structure, i.e., the magnetic field where the antiparallel coupling of the APC layer is overcome and the magnetizations of AP1 and AP2 become parallel. A high Hs, typically more than 5 kOe, is desirable to obtain a magnetically stable sensor.
What is needed is a CPP-SV sensor with increased magnetoresistance as a result of an improved AP-pinned structure but without a reduction in saturation field.